Release 3.1: Upgrade Requirements

Log file formats and the Btree, Queue, Recno and Hash Access Method
database formats changed in the Berkeley DB 3.1 release. (The on-disk
Btree/Recno format changed from version 7 to version 8. The on-disk
Hash format changed from version 6 to version 7. The on-disk Queue
format changed from version 1 to version 2.) Until the underlying
databases are upgraded, the DB->open() method will return a
DB_OLD_VERSION error.